1. Field of the Invention
This invention relates to a method for manufacturing a resin bound magnet. More particularly, it relates to a method for manufacturing a resin bound magnet having excellent strength and magnetic characteristics, and magnets produced therefrom.
2. Description of the Prior Art
In recent years, in response to an increasing demand for powerful and miniaturized magnets to be used for electric motors, a variety of resin bound magnets obtained using rare earth and transition metals having excellent magnetic characteristics have been proposed.
A resin bound magnet is normally composed of a powdered magnetic material and a binder resin material for binding the fine particles of the magnetic material. The powdered magnetic material is typically a combination of rare earth and transition metals, such as, samarium and cobalt (Sm-Co), or of neodymium, iron and boron (Nd-Fe-B), while the binder resin material is normally a solid epoxy resin (see Japanese Patent Publications, Tokkai Hei No. 1-114006 and No. 2-6573).
When a solid epoxy resin is used as binder, the resin bound magnet is typically prepared in the following manner. The solid epoxy resin is first heated and then dissolved in an organic solvent, such as, acetone containing a hardening agent. The powdered magnetic material is then added to this solution. Thereafter, the solution is mixed well and kneaded while dry nitrogen gas is blown into the solution to vaporize the solvent. The residue is then dried by using a vacuum dryer and the dried blocks of the compound are pulverized to powder form. The powder is passed through a sieve to produce particles with a substantially uniform granule size, which are then filled into a metal mold for compression molding. The molded body is then subjected to treatment to eliminate any powdery residue, heat-set the resin and other conventional supplementary treatments.
While a rare earth/transition metal type resin bound magnet prepared according to the above known method exhibits excellent magnetic characteristics, allows a number of poles to be formed, is easily worked by machine tools and can have a desired configuration, it is accompanied by the following problems due to the solid epoxy resin binder.
Firstly, since the solid epoxy resin is relatively insoluble in organic solvents at ambient temperature, it must be heated to accelerate its dissolution.
Secondly, because residual organic solvent in the resultant compound produces significant adverse effects on the radial compression strength of the compound after molding, the amount of the organic solvent in the compound must be rigorously minimized by allowing the powdered compound to remain under vacuum for 24 hours after a vacuum drying of 2 hours.
In addition, the amount of the powdered compound introduced into the compression metal mold can fluctuate depending on the solvent content of the compound. This results in inaccuracies in the amount of material introduced into the mold as well as the density and strength of the product. These problems, in turn, hinder the preparation of a resin bound magnet having a relatively large diameter, reduced thickness, or length, wherein the powdered compound must be very evenly filled with the mold. Moreover, since the prior art resin bound magnets did not exhibit sufficient radial compression strength, they could not be press fit into a rotor. Consequently, the magnets needed to be fitted to the rotor by using an adhesive agent. This resulted in reduced efficiency and increased costs.